Owing to the increasingly higher densities of integration in the arrangement of semiconductor elements, semiconductor exposure tools are being required to have higher resolution and deeper depth of focus. To achieve this, there is a trend towards use of shorter wavelengths, specifically, there has been a progressive shift towards shorter wavelengths for the light sources of semiconductor exposure tools, from the g line of high-pressure mercury lamps to the i line, and further, to KrF excimer lasers.
However, when the deep ultraviolet region, such as a KrF excimer laser (248 nm) or ArF excimer laser (193 nm) is employed as the generated laser beam, only a few types of optical material are suitable for use in the projection lens so that it is difficult to correct chromatic aberration. Consequently, in such excimer lasers, a monochromatic lens in which correction for chromatic aberration is not performed is employed as projection lens, and exposure tool light sources are employed in which monochromaticity is raised by narrowing the bandwidth of the excimer laser itself.
However, a narrow band laser apparatus in which all of the output light from the laser generating section 160 is input to a band-narrowing element as shown for example in FIG. 19 suffers from drawbacks such as that the laser beam output becomes small because of large losses in the band-narrowing element and that there are problems of durability due to the large load on the band-narrowing element. This trend is particularly marked in the case of a short-wavelength ArF narrow-band excimer laser. In FIG. 19, reference numeral 160 is the laser generating section, 161 is the band-narrowing element, 162 is a total reflection mirror, and 163 is a half mirror.
Accordingly, in the excimer laser technique illustrated in Japanese patent publication (kokai) No. 3-259583, an attempt to solve these problems is made by dividing the laser beam by a dividing mirror, inputting part of the divided laser beam to a band-narrowing element, and outputting remaining part as laser output: the construction is shown in FIG. 20-FIG. 21.
In FIG. 20, at one end of a laser tube 131 containing laser medium, there are provided a control plate 136 having a window 134 and slit 136a and a high-reflectivity mirror 135, while, at the other end of laser tube 131, there are provided a window 134, dividing mirror 137, first etalon 138, second etalon 139, high reflectivity fold-back mirror 141, beam splitter 142, dispersion plate 143, monitor etalon 144, condensing lens 145, linear line sensor 146, and oscilloscope 147.
In such a construction, some of the laser beam that is output from the left-hand window 134 is divided and reflected by dividing mirror 137 and is subjected to band-narrowing by passing through first etalon 138 and second etalon 139. The laser beam that has been subjected to band narrowing is reflected by high-reflectivity folding-back mirror 141 and dividing mirror 137 so that it is folded back to the laser excitation zone 132 in laser tube 131. After being amplified in laser excitation zone 132, the laser beam again passes through the same optical path as described above, but diverging with an angle .theta., before being emitted through left-hand window 134. Also, part of the laser beam that is emitted is reflected by dividing mirror 137 to be input once more to first etalon 138 and second etalon 139, the remaining part being output as output laser beam L. Part (about 1%) of output laser beam L is reflected at beam splitter 142 and input to linear line sensor 146, so that the intensity distribution of the laser beam can be monitored using the output of the sensor.
With this conventional laser apparatus, an output of high spectral purity containing few ASE (amplified spontaneous emission) components is sought to be obtained by avoiding the use of optical components such as beam expanders that cause generation of dispersed light, and the load on the band-narrowing element is sought to be reduced by arranging for the laser beam to be divided by a dividing mirror 137 and for the divided portion of the laser beam to be input to the band-narrowing element.
If an etalon is employed as band narrowing element and wavelength selection element, since the etalon itself is an element that selects the angle of incidence, an output beam of high spectral purity with little ASE component can be obtained without deterioration of the shape of the spectral distribution. However, if an etalon is used, the following problems arise in the case of a wavelength as short as that of an ArF laser (193 nm).
a. Although the load on the etalon can be reduced to some extent by a resonator as shown in FIG. 20, there are problems regarding durability. PA1 b. In order to make the spectral width less than 1 pm, it is necessary to provide a plurality of etalons, which raises the cost. PA1 c. Since the selected wavelength of an etalon fluctuates considerably due to heat, there are considerable difficulties in controlling a plurality of etalons such that they do not show thermal fluctuation. PA1 Since the optical path of the resonator is bent by dividing mirror 137, the resonator is unstable and resonates at a large number of wavelengths depending on vibration and thermal distortion. Angle-dispersion type elements are particularly subject to vibration. PA1 Since beam division is effected by utilizing the edge portion of dividing mirror 137, if the planar accuracy of the edge portion is poor, the wave surface of the beam enters the band-narrowing element in a distorted condition, adversely affecting the shape of the spectral distribution.
Thus, for the band narrowing element and wavelength selection element, rather than elements such as etalons, which are not of angle-dispersion type, it is more advantageous in regard to solving the above problems to employ angle-dispersion type band-narrowing and wavelength selection elements.
FIG. 21 shows another embodiment illustrated in Japanese patent publication (kokai) No. 3-259583 referred to above. In this conventional laser apparatus, instead of the first etalon 138, second etalon 139 and high reflectivity fold-back mirror 141 used to constitute the band narrowing element in FIG. 20, an angle-dispersion type band narrowing element constituted by prism beam expander 156 and diffraction grating 157 is used.
However, if an angle-dispersion type band narrowing element is employed, since an angle-dispersion type band narrowing element diffracts the light in all directions in the plane of the dispersion direction, a large number of parasitic oscillations are produced as shown by the dotted line in FIG. 2. This had a severely adverse effect on the shape of the spectral distribution of the beam.
Also, there are the following problems common to the techniques shown in FIG. 20 and FIG. 21, which are disclosed in Japanese patent publication (kokai) No. 3-259583.
FIG. 22 shows another conventional technique wherein a resonator construction is adopted such as to reduce the load on the band-narrowing element (Japanese patent publication (kokai) No. 2-213178).
In this conventional technique shown in FIG. 22, a totally reflecting mirror 152 is provided on one side of laser discharge tube 151 and a totally reflecting mirror 153 is provided on the other side, a semi-transparent mirror 154 being arranged between laser discharge tube 151 and totally reflecting mirror 152. Thus, in using the light reflected by semi-transparent mirror 154 as output beam 156 for semiconductor exposure etc., band narrowing of the laser beam can be achieved by using grating 155 to narrow the band of the light passing through semi-transparent mirror 154, then returning the beam to within laser discharge tube 151.
However, with this conventional technique, since only part of the light reflected by totally reflecting mirror 152 after band narrowing by grating 155 passes through semi-transparent mirror 154 and is thereby folded back to laser discharge tube 151 and amplified, the rest of the light being reflected in the opposite direction to laser output beam 156 and discarded, there is a waste of light energy.
If an excimer laser is employed as light source of a semiconductor exposure tool, as well as narrowing the laser bandwidth, it is necessary to obtain large output by effectively utilizing the optical energy generated in the interior of the discharge tube, so such waste of part of the beam has to be avoided. Thus, with the conventional narrow-band laser device, there were problems concerning the durability of the band narrowing element that generation of a lot of parasitic oscillations produced distortion of the shape of the spectral distribution, the resonator was unstable, and also drawbacks such as high cost, liability to be effected by thermal fluctuation and waste of optical energy.
It is an object of the present invention to provide a narrow band laser apparatus wherein the above-mentioned drawbacks are improved, the spectral distribution of the laser beam is well-formed, which is stable and has excellent durability and wherein energy is not wasted.